1. Field of the Invention
The present invention relates to the field of mobile telephony short message service (SMS). In particular, to an apparatus and a method for improving short message service dependability.
2. Background Art
FIG. 1 is a schematic representation of an exemplary short message service (SMS) network architecture 100 illustrating message flows.
In the above short message service network architecture both a router 110 and a short message service center (SMSC) 120 have Signaling System 7 (SS7)/Internet Engineering Task Force (IETF) SIGTRAN interfaces to a public land mobile network (PLMN) 130. The router 110 and the SMSC 120 communicate with each other using a standard short messaging stack (for Global System for Mobile Communications (GSM) networks this is defined in GSM 03.40 & Mobile Application Part (MAP), for 3rd Generation Partnership Project (3GPP) by European Telecommunications Standard Institute (ETSI) 23.040 and MAP and for code division multiple access (CDMA) networks it is defined in the IS-41 specification). An interface between the router 110 and the SMSC 120 frequently uses the same stack as that used to connect each of the router 110 and the SMSC 120 respectively to the mobile network 130. Alternatively the interface between the router 110 and the SMSC 120 can use IP based Short Message Peer to Peer Protocol (SMPP) defined in the SMPP Developer Forum's Short Message Peer to Peer Protocol Specification v3.4.
Mobile originated (MO) short messages (SM) are handled initially by the router 110. Typically 80% or more of mobile terminated (MT) SM are delivered at the first attempt so the router 110 will immediately attempt to deliver the SM. If this delivery attempt fails then the router 110 passes the SM to the SMSC 120. The SMSC 120 stores the SM and at a later time makes further attempts to deliver it.
MO SM are handled initially by the router 110. Typically in excess of 95% of fixed terminated (FT) SM are delivered at the first attempt so the router 110 will immediately attempt to deliver the SM via a gateway 140. If this delivery attempt fails then the router 110 passes the SM to the SMSC 120. The SMSC 120 stores the SM and at a later time makes further attempts to deliver it.
Fixed originated (FO) SM are initially handled by the gateway 140. If an application server 150 that originated the FO SM selects single shot quality of service then the SM is forwarded directly to the router 110 for delivery. If store and forward quality of service is selected then the gateway 140 forwards the SM to the SMSC 120 instead.
Alternatively the gateway 140 can send all FO-MT SM to the router 110 for an initial delivery attempt, and if this fails the router 110 can forward them to the SMSC 120 for storage and forwarding.
The SMS network can comprise one of more of each of the router 110, the SMSC 120 and the gateway 140 to, for example, to increase the SMS traffic handling capacity. Individual router 110, SMSC 120 and gateway 140 may be group to provide one service or alternatively all of the routers 110, SMSC 120 and gateways 140 may to cross-connected in a network configuration.
When first launched in the 1990's the SMS was strictly a store and forward service. SM were stored persistently in an SMSC 120 and then delivery was attempted. This meant that once an SM had been acknowledged to the submitter it was secured to persistent storage and nothing short of the complete failure of the SMSC 120 or multiple disk failure could result in its loss. This approach is secure and helps to ensure accurate real time charging, but it drives up the cost of the SMSC 120 because expensive storage subsystems are needed to achieve even moderate throughputs. The store and forward based reliability mechanism established the sense of dependability of the SMS in consumers' (i.e. users) perception.
Over time the volume of SM sent has increased by several orders of magnitude, and the unit price of an SM has dropped significantly. Because profit margins are now lower operators have sought to reduce costs when increasing their SMS network capacity. Many networks no longer offer a true store and forward service, but instead offer a less dependable alternative commonly referred to as forward and store. Forward and store is where an attempt is made to deliver the SM (hereafter referred to as the First Delivery Attempt or FDA) before writing it to persistent storage and only storing it if the FDA attempt fails. Clearly if a high proportion of these delivery attempts are successful (which is the norm in a modern network) this approach significantly reduces the number of SM that need to be stored, and thus reduces the cost of a SMSC platform for a given capacity. Superficially the risk involved in the forward and store quality of service does not appear great because theoretically the SMSC can defer acknowledging the SM until after it has safely stored it. However in practice this approach is only viable for MO-FT SM and some FO-MT SM, not for the majority case—MO-MT. The reason for this is that when a mobile handset originates an SM it starts a timer. If the SM is not acknowledged before this timer expires then the handset automatically retransmits the SM. In a typical GSM network approximately 5% of all delivery attempts take longer to complete than the timeout period of the timer. This means that for MO-MT SM the SMSC 120 must acknowledge receipt of the SM before it has stored it in at least some cases in order to avoid duplicate delivery of the SM to its recipient. The acknowledgement of the SM before it has been stored is known as ‘early acknowledgement’ (Early-Ack) as described in United Kingdom patent GB2401757B issued Jun. 10, 2005. If a component of the SMSC 120 fails between sending the acknowledgement and storing the SM then not only is the SM lost, but the submitter (i.e. originator) is unaware of this because the SM has been acknowledged to them. These lost SM reduce subscriber confidence in what was previously seen as a highly reliable service.
For some years now some network operators have had SMS routers (therein after routers) in their networks. Initially these systems were only used to load balance traffic between multiple SMSC 120 and as such did not alter the dependability of the service. Recently the majority of routers 110 have acquired the ability to offer the forward-and-store quality of service described above (i.e. FDA and forward to the SMSC 120 if the FDA fails). This approach significantly increases the risk of failure when compared with the store-and-forward implementation described above (internal to the SMSC 120). In the architecture having a router 110 providing forward-and-store functionality, the router 110 acknowledges the SM to the submitter and then attempts delivery. If this delivery attempt fails the router 110 then forwards the SM to the SMSC 120 that stores it. There are three potential points of failure after the receipt of the SM has been acknowledged:                The router 110 itself could fail before forwarding the SM to the SMSC 120.        The communications network between the router 110 and the SMSC 120 could fail.        Finally the SMSC 120 could fail before securing the SM to disk.        
If a router 110 is running a pure forward and store service at a traffic rate of 2,000 SM/second and the typical time to deliver an SM is 10 seconds then up to 20,000 previously acknowledged (and potentially charged for) SM will be lost if the router 110 fails.
In some countries there are statutory requirements on the accuracy of telephony billing systems. In situations where SMs are charged for in real time (for example pre-pay phones) the combination of the issues listed above may put operators at risk of exceeding their permitted number of charging errors. The degree of risk depends on the exact mechanism used for real time charging.
The forward and store quality of service also potentially weakens the ability of an SMSC 120 to perform the duplicate checks intended to allow it to identify retransmissions. This is a result of the SMSC 120 potentially having no record of the SM that it has delivered without storing them and therefore the SMSC 120 has nothing to compare new SM to in order to determine if they are duplicates.